I. Field of the Invention
The present invention relates generally to communication systems for secure transmission of audio signals and more particularly to a method for synchronizing the scrambling sequences of communicating scrambler units in a reliable and secure manner.
II. Description of the Prior Art
Communication systems have been developed which serve to prevent unauthorized persons from intercepting and acquiring the transmitted intelligence communicated therefrom. These systems are based upon a psuedo-random scrambling of the speech signals prior to transmission so as to render the transmission unintelligible and therefore secure with respect to unauthorized third parties who may intercept the transmission.
Two general approaches have been developed for enciphering speech. The first approach contemplates converting the analog speech signals into digital signals and linking the digital pulses in conventional manner with one another by means of key pulses which are generated by a key generator. The thus enciphered characters are transmitted to the receiver end and then converted into deciphered analog speech. However, this type of prior art system requires a large bandwidth for transmission purposes and the equipment is sensitive to phase shifts in the transmission system.
The second prior art approach does not transform the speech signals into digital form. The speech information is subdivided into partial groups along the frequency axis or time axis. These partial groups are then permuted by key information generated by a key generator so that there is produced a new sequence of the partial groups. Yet the information as such is still accommodated within the same frequency band thus permitting a narrowband scrambling system. However, it has been found that the simple transposition of sub-groups provides insufficient security against deciphering. Accordingly, some of these narrowband systems periodically change the scrambling algorithm and then transfer coded key information for descrambling between communicating scrambler units to provide greater security against deciphering. Such prior art systems transmit coded key information at the beginning of a transmission and the receiving unit uses this information to descramble the transmission. This approach is unreliable in noisy environments or in environments where fading is common. In addition, these systems do not permit late but authorized entry into the system by a third party.